The invention relates to hydraulic lift mechanism and in particular to such mechanism as is required to serve intermittent alternating vertical displacement of a load, wherein the load may be of various magnitudes within the capacity of the mechanism. Such conditions exist for hydraulically operated cranes and hoists, and the present invention is particularly concerned with dual-lift systems for coordinated handling of the respective ends of an elongate load, exemplified by a freight container within which the loading may not be uniformly distributed, thus presenting a different lifting-load demand on the respective lifts of the dual system. For simplicity of discussion herein, all references are to such a container as the load, in which case a conventional "spreader" device is to be understood as the means whereby cable-derived lift is transmitted to the container, via releasable engagement of the spreader to the four corners of the container; the word "container" is not to be understood as limiting, since the invention is applicable to the handling of other kinds of load wherein lift requirements differ at longitudinally spaced lift points of the load.
Conventional cranes and hoists of the character indicated require two independent lift systems, one for handling each of the respective ends of the container. The prime mover for each of the two lift systems is a diesel engine or one of various types of electric motor, depending upon the design capacity of the involved lift system, and the rated power of each prime mover is conservatively selected for assured handling of the maximum rated load. In most cases, each of the two lift systems requires its own gear box, speed reducer, pulling drum (or winch) and safety brake; and the operator must exercise extreme skill in coordinating the functions of these components for both systems at the same time. Illustratively, a conventional container crane with a 30-ton capacity having a 30-horsepower prime mover for each of its lift systems is able to handle its maximum load at a lifting speed of 7 feet per minute, and when weight distribution is so uneven as to present different lift demands to the respective lift systems, this lifting speed must suffer, due to the complexity of requisite operator-coordinated control.